标题： 中等 standoff 距离下气泡溃灭导致的空化侵蚀新认识 显示英文标题

标题： New insights into the cavitation erosion by bubble collapse at moderate stand-off distances

摘要： 非球形气泡在靠近固体边界处的塌缩会产生水锤压力和冲击波，这些被公认为是空化侵蚀的关键机制。然而，对于局部侵蚀模式尚未达成一致意见，空化侵蚀损伤也缺乏定量分析。 在我们的实验中，在铝试样表面发现了五种明显的局部侵蚀模式，这是由于在中等悬空距离（$0.4\le\gamma\le2.2$）下激光诱导的空化气泡塌缩所导致的，分别为双极型、单极型、环状型、太阳晕型和中心型。 其中，当环形气泡在第二周期沿周向发生不对称塌缩时，双极型和单极型表现出最严重的空化侵蚀。 阴影可视化显示，不对称塌缩通过正面碰撞和波前斜交叠加导致冲击波聚焦。 这导致在某些悬空距离下，环形气泡半径的变化和最大侵蚀深度的位置并不匹配。 初始等离子体的不对称性和气泡壁的悬空距离都是决定周向不对称塌缩行为的关键因素。 在较大的初始纵横比下，椭圆射流尖端在收缩过程中形成，导致环形气泡从曲率半径较小的区域塌缩，并最终沿周向汇聚到碰撞点。 我们使用OpenFOAM进行的三维模拟成功再现了周向不对称气泡塌缩的关键特征。 本研究为非球形近壁气泡塌缩动力学提供了新的见解，并为开发空化侵蚀预测模型奠定了基础。 显示英文摘要

摘要： Non-spherical bubble collapses near solid boundaries, generating water hammer pressures and shock waves, were recognized as key mechanisms for cavitation erosion. However, there is no agreement on local erosion patterns, and cavitation erosion damage lacks quantitative analysis. In our experiments, five distinct local erosion patterns were identified on aluminum sample surfaces, resulting from the collapse of laser-induced cavitation bubbles at moderate stand-off distances of $0.4\le\gamma\le2.2$, namely Bipolar, Monopolar, Annular, Solar-Halo, and Central. Among them, the Bipolar and Monopolar patterns exhibit the most severe cavitation erosion when the toroidal bubbles undergo asymmetrical collapse along the circumferential direction during the second cycle. Shadowgraphy visualization revealed that asymmetrical collapse caused shockwave focusing through head-on collision and oblique superposition of wavefronts. This led to the variations in toroidal bubble radii and the positions of maximum erosion depth not matching at certain stand-off distances. Both initial plasma asymmetry and bubble-wall stand-off distance were critical in determining circumferential asymmetrical collapse behaviors. At large initial aspect ratios, the elliptical jet tips form during the contraction process, resulting in the toroidal bubble collapsing from regions with smaller curvature radii, ultimately converging to the colliding point along the circumferential direction. Our three-dimensional simulations using OpenFOAM successfully reproduce the key features of circumferentially asymmetrical bubble collapse. This study provides new insights into the non-spherical near-wall bubble collapse dynamics and provides a foundation for developing predictive models for cavitation erosion.